I am using a 2012 MBPr 15" running the latest version of Mavericks (OS X v10.9).
The fullscreen functionality when using CrunchyRoll with Safari does not work properly as shown in the picture attached. The problem only persists in Safari and does not occur in Firefox on the same machine.
I know Mavericks has plenty of problems so I wouldn't be surprised if it was a problem with the OS, but probably a good idea to give you a heads up that there might simply be a browser compatibility issue that needs to be resolved.
EDIT: My apologies, but the same problem appears to be posted already except the problem persists in Chrome. http://www.crunchyroll.com/forumtopic-826309/fullscreen-only-part-of-screen